Inorg. Chem. 1992, 31, 3230-3235
3230
Excited-State Properties of Platinum(I1) Complexes Containing Biphenyl as a Ligand: Complexes of the Type [(bph)PtLz], Where L = Monodentate or Saturated Bidentate Ligands Charles B. Blanton, Zakir Murtaza, Randy J. Shaver, and D. Paul Rillema' Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223
Received October 23, 1991 The preparation, physical properties, and photophysical properties of [Pt(bph)(C2H5)zS]zl [Pt(bph)(CH3CN),], [Pt(bph)(en)], and [Pt(bph)(py)z], where bph2- is the dianion of biphenyl, en is ethylenediamine, and py is pyridine, are described. The complexes display d a A* transitions in the 300-400-nm region and intraligand A A* transitions at higher energy. Only one redox process, an irreversible oxidation, is observed in the +2 to -2 V region. The Ell2 values range from 0.67 to 1.35 V vs SSCE. Highly structured emission is observed and attributed to a 3LC IGS process. Spectral curve fitting using a four-mode fit gives Em = 20 670 cm-l and vibrational frequencies of 1550, 1150, 715, and 375 cm-1 that are nearly independent of the specific complex. Temperature-dependent lifetime data reveal the presence of an energy level above the 3LC state with an activation energy of 1220 cm-1 for [Pt(bph)(CH3CN)2],2280 cm-I for [Pt(bph)(py)~],and 2490 cm-I for [Pt(bph)(en)]. The energy gap of -2.6 eV gives rise to high oxidative quenching power (-1.14 to -1.71 V) demonstrated by electron-transfer quenching with methyl viologen and o-nitrobenzene.
-
-
+
Introduction
emitters were [Pt(diimine)(biphenyl)] complexes6 and Pt(I1) complexes containing 2-phenylpyridine and 2-(2-thionyl)pyriThe search for inorganicmaterials that would behave as energydine The final classificationconsistsof complexes that or electron-transfer photocatalysts in solution has primarily emit from metal-metal excited states. Complexesdemonstrating involved investigating the photochemical and photophysical this property were [Ptz( P z O ~ H Z4? ) ~ ][Pt (CN)4].2-,10 P t 2 [bisproperties of ruthenium(II), osmium(II), and rhenium(1) com(diphenylphosphine)methane]~(CN)~] and an excimer based plexes.' Platinum(I1) complexes have received less attention, on [Pt(4,7-diphenyl-l, lO-phenanthr~line)(CN)~] -12 but in the few cases published, platinum(I1) complexes have been In this report, we examine the physical and photophysical shown to have promising properties.z-12 A number of these properties of a new series of Pt(I1) complexes containing the complexes luminesced strongly in solution, some underwent biphenyl dianion as a ligand, which was expected to result in excited-state electron-transfer quenching, and some displayed high-energy emission, provided emission occurred from the a* photoassisted oxidative addition reactions. energy level of the ligand. To ensure this, two synthetic strategies These platinum(I1) complexes can be divided into four types were used: (1) The bph ligand was coordinated in a bidentate on the basis of their emitting states. Those classified as intralifashion through the carbon atoms located in the 2,2' positions of gand (IL) A A* emitters were Pt(I1) complexes of 8-quinolinol biphenyl; (2) the other ligand(s) chosen either had higher A* chelates,2 a [Pt(5,5'-dimethyL2,2'-bipyridine)(CN)21 a d d ~ c t , ~ energy levels than bph (pyridine, acetonitrile, diethyl sulfide) or and a [Pt(2,2'- bipyridine)(NH3)2]2+-dibenzo[301crown-10 adwere saturated alkylamines (ethylenediamine). We reported our Those classified as interligand (LLCT) r a* emitters initial success following this line of attack earlier.13 werePt(II)dithiolate4iiminecomplexes.~T h e d r + r * (MLCT) Experimental Section +
-
General reviews: (a) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1192. (b) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988,84,85. (c) Lees, A. J. Chem. Rev. 1987,87,711. (d) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg. Chem. 1986, 25, 3858. Che, C.-M.; Wan, K.-T.; He, L.-Y.; Poon, C.-K.; Yam, V. W.-W. J . Chem. SOC.,Chem. Commun. 1989,943. Ballardini, R.; Gandolfi, M. T.; Balzani, V.; Kohuke, F. H.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1988, 27, 692. Sykora, J.; Sima, J. Coord. Chem. Rev. 1990, 107, 1. (a) Cornioley-Deuschel, C.; von Zelewsky, A. Inorg. Chem. 1987, 26, 3354. (b) Maestri, M.; Sandrini, D.; Balzani, V.; von Zelewsky, A.; Deuschel-Cornioley, C.; Jolliet, P. Helv. Chim. Acta 1988, 71, 1053. (a) Maestri, M.; Sandrini, D.; Balzani, V.; Chassot, L.; Jolliet, P.; von Zelewsky, A. Chem. Phys. Lett. 1985, 122, 375. (b) Barigelletti, F.; Sandrini, D.; Maestri, M.; Balzani, V.; von Zelewsky, A.; Chassot, L.; Jolliet, P.; Maeder, U. Inorg. Chem. 1988, 27, 3644. Craig, C. A.; Garces, F. 0.;Watts, R. J.; Palmano, R.; Frank, A. J. Coord. Chem. Rev. 1990, 97, 193. Roundhill, D. M.; Gray, H.G.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55 and references therein. (a) Schindler, J. W.; Fukuda, R. C.; Adamson, A. W. J.Am. Chem. SOC. 1982, 104, 3596. (b) Lechner, A.; Gliemann, G. J . Am. Chem. SOC. 1989, I I I, 7469 and references therein. Che, C.-M.; Yam, V. W.-W.; Wong, W.-T.; Lai, T.-F. Inorg. Chem. 1989, 28, 2908. Kunkely, H.; Vogler, A. J. Am. Chem. SOC.1990, 112, 5625.
0020-1669/92/1331-3230$03.00/0
Materials. Acetonitrile and methylene chloride were spectrophotometric grade and were dried over 4-A molecular sieves before use in electrochemistry experiments. Tetrabutylammonium hexafluorophosphate (TBAH) was purchased as electrometric grade from Southwestern Analytical and stored in a vacuum oven before use. n-Butyllithium in hexane was provided by Lithco, Inc., a division of FMC. Reactions involving lithium alkyl reagents were carried out under a nitrogen atmosphere and handled by way of syringe techniques. Potassium tetrachloroplatinate was obtained on loan from Johnson-Matthey, Inc. All other reagents were purchased as reagent grade and were used without further purification. Preparationof Compounds. The following compounds were prepared according to literature procedures: 2,2'-dibromobiphenyl (mp 77-78 "C (lit. 78-79 "C)),I4 2,2/-dilithiobipheny1,'5 [P~CI~((CZH~)ZS)Z] ,I6and [Pt(bph)(CzHs)2S]~.~~ The elemental analysis results for [Pt(bph)(C2H5)2S]2were as follows: Anal. Calcd for P ~ ~ C ~ Z HC,~43.93; ~ S ZH, : 4.15; S,7.33. Found: C, 43.88; H, 4.17; S,7.27. [Pt(bph)(ethylenediami~te)]. A 25-mL round-bottom flaskwas charged mol) of [Pt(bph)(CzHs)2S]2 and 0.020 mL of with 22 mg (2.55 X (13) Blanton, C. B.; Rillema, D. P. Inorg. Chim. Acta 1990, 168, 145. (14) Gilman, H.; Gaj, H. J. J. Org. Chem. 1957, 22,447. (15) Gardner, S. A.; Gordon, H. B.; Rausch, M. P. J . Organomet. Chem. 1973, 60, 179. (16) Kaufmann, G. B.; Cowan, D. 0. Inorg. Synrh. 1960, 6, 211.
0 1992 American Chemical Society
Inorganic Chemistry, Vol. 31, No. 15, 1992 3231
(Biphenyl)platinum(II) Complexes Table I. Visible/UV Spectral and Redox Properties
E112' complex [Pt(bph)CtHs)~SI2~.~ [Pt(bph)(PY)ZI [Pt(bph)(CH0hle [Pt(bph)(en)ld
dr
+
[Pt(bph)L2]'10
[Pt( bph)L2]+/or
1.35 (irr)
-1.14
0.81 (irr)
-1.85
261 (1.4 X lo4) 258 (4.8 X lo4)
0.85 (irr)
-1.67
258 (4.8 X lo4) 233 (1.7 X lo4)
0.67 (irr)
-1.71
a*
340 (2.3 X lo4) 310 (2.4 X lo4) 375 sh 332 (4.0 X lo3) 303 (3.8 X lo3) 380,357 sh 326 (1.6 X lo4) 302 (1.3 X lo4) 380,357 sh 333 (6.0 x 103) 305 (4.3 x 103)
A+
a T = 23 f 1 'C; X in nm, error = f l nm; e in M-I cm-I, error = f O . l . irr = irreversible. c Reference 6a. d In methylene chloride. e In acetonitrile.
diethyl sulfide dissolved in 15 mL of methylene chloride. To this stirred platinum solution was added dropwise 100 mg (1.50 X lo-) mol) of ethylenediamine in 5 mL of methylene chloride. Immediately, the color changed from a light yellow to a deep yellow. Approximately 5 min later, a yellow-green precipitate began to form. The suspension was protected from light and was stirred for 30 min under nitrogen. A yellow-green luminescencewas observed in the solid state. The precipitate was collected and washed with diethyl ether. Yield = 8 mg (50.6%). Anal. Calcd: C, 41.28; H, 3.96; N, 6.88. Found: C, 41.11; H, 4.00; N, 6.83. [Pt(bph)(pyridine)z]. A 50-mL round-bottom flask was charged with 19mg(2.12 X l t 5 m o l ) of [Pt(bph)(C2H~)2S]2andO.O30mLofdiethyl sulfide dissolved in 15 mL of methylene chloride. A solution of 0.80 mL (9.90 X l e 3 mol, d = 0.978 g/mL) of pyridine in 7 mL of methylene chloride was added dropwise to the stirred platinum solution. A yellow precipitate began to form midway through the addition. The suspension was protected from light and allowed to stir for 30 min. The precipitate was collected by vacuum filtration and washed with copious amounts of diethyl ether and hexane to remove the unreacted pyridine. A yellowgreen luminescence was noted in the solid state. The product was then dried in a vacuum oven overnight. No further purification was required. Yield = 16 mg (74.3%). Anal. Calcd: C, 52.27; H, 3.59; N, 5.54; S, 0.00. Found: C, 52.00; H, 3.65; N, 5.46; S, 0.00. [Pt(bph)(CH3CN)z]. A 50-mL round-bottom flask was charged with 10 mg (1.14 X mol) of [Pt(bph)(C2H&S]2 and 20 mL of acetonitrile. This mixture was sonicated until a clear yellow solution was obtained. The solvent was removed by rotary evaporation, and a yellow solid was obtained. A yellow-green luminescence was observed in the solid state. No further purification was performed. Yield = 9 mg (90%). Anal. Calcd: C, 44.76; H, 3.29; N, 6.52; S, 0.00. Found: C, 44.95; H, 3.42; N, 6.17; S, trace. Measurements. Visible/UV spectra were obtained with a PerkinElmer Model 3840 diode array spectrophotometer. Corrected emission spectra were recorded with a Spex Fluorolog Model 212 spectrofluorometer. Cyclic voltammograms were obtained using a three-electrode system consisting of a Pt-disk working electrode, a Pt-mesh counter electrode, and a saturated sodium chloride calomel electrode (SSCE). The electrochemical measurements were made with a PAR 174 polarographic analyzer or a PAR 173 potentiostat in conjunction with a PAR 175programmerandrecordedwithaYEW Model3022A4X-Y recorder. Excited-state lifetimes were acquired using a Photochemical Research Associates (PRA) Model LNlOOO pulsed N2 laser and a PRA LN102 dye laser. The measurements were carried out with a LeCroy transient digitizer system employing either a 6880A or a TR 8828C transient recorder. The LeCroy 6010 controller was interfaced to an IBM PS/2 Model 60 microcomputer via a National Instruments GPIB IEEE-488 bus and controlled by Catalyst software supplied by LeCroy. Computations of time-dependent emission decays were performed by an inhouseprogram,LrFeTiMEwritten by Randy J. Shaver,utilizinganalgorithm designed to fit the data to a single-exponential decay. Temperaturedependent excited-state lifetime measurements were obtained by utilizing a Cryo Industries Model EVT cryostat controlled by a Lakeshore 805 temperature controller. A Photon Technology, Inc., apparatus consisting of a PTI LPS 200X 74-W photolysis system, an Oriel Model 77250 monochromator, and a cell holder mounted on an optical rail was used for photolysis investigations. The cells were equilibrated at a temperature of 25 1 OC in a Haake
*
A*
266 (3.7 x 252 (3.8 X 235 (4.6 X 275 (3.3 x 236 (1.1 x
104)
lo4) lo4) 104) 104)
T = 23 f 1 OC; E l p in V vs SSCE, error = f0.02V; 0.1 M TBAH;
FK2 constant-temperature bath. The light intensity was determined at 355 nm using ferric oxalate" as the actinometer. A neodymium-YAG laser system located at the University of North Carolina at Chapel Hill was used for laser flash photolysis measurements. Computer fits of temperature-dependent lifetime data were obtained with an IBM PS/2 Model 60 microcomputer utilizing either an algorithm based on a nonlinear-least-squares analysis or an algorithm based on procedures outlined by Wentworth18 which allows for variance in both plotted variables. The low-temperature emission spectral fits were performed with a simple algorithm based on a multimode vibrational analysisI9 and modified by Dr.Cliff Carlin of our department for anharmonicity of the vibrational modes. Elemental analyses werecarried out by Atlantic Microlabs, Inc., Norcross, GA.
Results Preparation of Complexes. [Pt(bph)(CzH5)2S]z served as the startingmaterialin thesynthesis of [Pt(bph)Lz] and [Pt(bph)(LL)] complexes, where L = acetonitrile (CH3CN) and pyridine (py) and L-L = ethylenediamine (en). The synthesis involved addition of (C2H5)2Sto the starting material in methylene chloride followed by addition of the desired ligand to yield the reported complexes with one exception. Thermal displacement of ( C ~ H S ) ~ S by CH3CN was effected in acetonitrile without added (CZH5)2S. Presumably, added (C2H5)2Sfacilitates formationof a monomer/ dimer equilibrium, [Pt(bph)(CzHs)zS]2 + 2(CzH5)zS F! 2[Pt(bph)(C2Hs)2S)z], where the monomer serves as the reactive intermediate. Electronic Spectra and Redox Behavior. The absorption features, absorption coefficients, and redox properties of the complexes are listed in Table I. The low-energy transitions in the 300400-nm region were assigned as d a d a*(bph) transitions by analogy to previous transition assignments made for [Pt(bph)( b ~ y ) ] . ~Higher a energy transitions which ranged from 235 to 270 nm were also observed. The transitions were assigned as a a* intraligand transitions and have molar absorptivities greater than IO4 M-I cm-*. In the solvent window from +2 to-2 V, the complexes displayed one irreversiblecouple that ranged from 0.67 to 1.35V, depending on the complex. Similar couples in other platinum compounds have been assigned to the oxidation Pt(I1) Pt(III).20 The oxidized species then underwent reaction, presumably with solvent, to yield an unidentified product which was observed to plate out on the platinum electrode. Luminescence Properties. The luminescence spectra of the complexes in solution are illustrated in Figure 1. The spectra show vibrational structure with maxima located at 493,532,569,
-
-
(17) Allmand, A. J.; Webb, W. W. J . Chem. SOC.1929, 1518. (18) Wentworth, W. E. J . Chem. Educ. 1965, 42.96, 162. Chapel (19) Lumpkin,R.S. Ph.D.Thesis,TheUniversityofNorthCarolinaat Hill, 1987, and references therein. (20) von Zelewsky, A.; Gremaud, G. Helv. Chim. Acro 1988, 84, 85.
Blanton et al.
3232 Inorganic Chemistry, Vol. 31, No. 15, 1992
Table 11. Emission Energy Maxima, Emission Lifetimes, and
8.3e*06r
Emission Quantum Yields" 298 K
77 K
70,E'd
Xmax,b.t
complex
nm
ps
nm
ps
[Pt(bph)(CzHsSIz [Pt(bph)(CH3CN)]zh [Pt(bph)(en)l [Pt(bPh)(PY)ZI
497 493 493 492
10.5
490 489 489 488
12.28 15.7' 14.4'
Xmax,b.C
14.0 7.0 3.2
lo$+
7.U
bcmCJ 0.0077 0.18 0.16 0.15
a In methylene chloride unless otherwise noted; &, = 355 nm. Error in A,, = *1 nm. T = 298 f 1 K. Error in 70 = &IO%. T = 77 K. /Radiative quantum yield error = *5%. 8 In methylene chloride-ethanol glass. In acetonitrile. In butyronitrile glass. J In isopentene-ethanol-diethyl ether-pyridine glass.
*
lifetimes did increase somewhat upon lowering the temperature to 77 K, resulting in a range from 8 to 16 1 s . The emission Wavelength (nm) quantum yields were on the order of 0.16 f 0.01, except for that Figure 1. Corrected luminescence spectra of complexes in methylene of [Pt(bph)(C2Hs)zS]z, which was 0.0077. This lowering may = 355 nm): (1) [Pt(bpy)(en)]; (2) chloride at room temperature (b,,, be due to the fact that it is a bimetallic complex and, in cases [Pt(bpy)(CH3CH)~l;(3) [Pt(bph)(p~)21;(4) [ P ~ ( ~ P ~ ) ( C Z H S ) ~ S I Z * where luminescencehas been observed for bimetallic complexes, the emission quantum yields have been lowered relative to their 2 . 7 ~ 6 monometallic analogues,21 presumably due to the heavy-atom effect, which provides additional low-energy vibronic pathways for excited-state energy loss. The luminescencewas quenched in the presence of the C1- ion. The quenching process was a static one, suggesting that new species were formed by way of a dark reaction. Upon addition of C1- to [Pt(bph)(~y)~] in methylene chloride, the d r r* transition decreased in intensity and broadened somewhat and then a slow reaction followed, as noted by a decrease in absorbanceat 332 nm over a 4-h period. Details regarding this reaction are currently under investigation. Electron-Transfer Qwacbiag. The complexes behaved as powerful photoreductants and followed the behavior illustrated in e q 1-3, where Q is an electron acceptor, k, is the quenching
E
n
-
-
hv, 355 nm
Figure 2. Dependence of excitation energy on emission wavelength for [Pt(bph)(C2H&S]2 in methylene chloride at 25 1 OC.
and 620 nm. (There was a 4-nm red shift for the maxima of the sulfur-bridged dimer.) The vibrational progressions, which correspond to ring stretching modes, were approximately 1260 cm-1 apart. A solvent dependency study proved unfeasible due to the limited solubility of the complexes in various solvents but would probably not have provided additional insight as noted by the overlay in Figure 1, which consists of spectra for [Pt(bph)( p ~ ) in ~ ]methylene chloride and [Pt(b~h)~(CH$N)zlin acetonitrile. Thus, on the basis of the similarity of the two spectra, it seems a reasonable conclusion that a change in solvent would not cause a significant deviation in either the position or the shape of the emission spectrum. The luminescence spectra at 77 K were basically the same as thoseat room temperature. Littlechangein theshapeor intensity from the room-temperature results was noted, except there was an approximate 4-nm blue shift of the spectra. In addition to the vibrational structure noted above, new structure was observed with progressions of about 300 cm-l. Excitation spectra of the complexes are illustrated in Figure 2. The spectra are basically the same with maxima located near 300 nm corresponding to the d r r*(bph) transition. The excitation spectra were independent of the observation emission wavelength, indicating that emission occurs from one excited state in the complexes. The high-energy emission maxima, excited-state lifetimes, and emission quantum yields are listed in Table 11. The excited-state lifetimes, determined at room temperature, ranged from 3 ps for [Pt(bph)(py)2] to 14 ps for [Pt(bph)(CH,CN)z]. The emission
-
[Pt(bPh)L,l
[Pt(bPh)L,l*
(1)
rateconstant, and kat is the back-electron-transfer rateconstant. Estimates of excited-state redox potentials are given in Table I. The quencher used for reactions with [Pt(bph)(CH$N)z] and [Pt(bph)(en)] was methyl viologen (E1/2(MV2+/+)= -0.40 V).22 The quencherz3used for the reactions with [Pt(bph)(CzH~);rS]z and [Pt(bph)(py)z] was o-dinitrobenzene (E1p(Q0/-)= -0.81 V).Z4 The results of Stern-Volmer studies based on luminescence quenching are presented in Table 111. Stern-Volmer constants (Ksv) ranged from 19 690 to 56 107 M-l. The quenching rate constants k,, where k, = K s v / T o , ~were ~ diffusion controlled. Laser flash photolysis provided spectroscopic evidence for MV+ (A, = 605 nm, c = 13 700 M-1 cm-1).26 The back-electrontransfer rate constant, kbct, for the reaction of [Pt(bph)(CHjCN)z]+ with MV+ had a value of 1.5 X 1O1OM-1 S-I. (21) Fuchs, J.; Lofters. S.;Dieter, T.; Shi, W.; Morgan, R.; Strekas, T. C.; Gafney, H. D.; Baker, A. D. J . Am. Chem. Soc. 1987, 109, 2691. (22) Bard, A. J.; Lund, H. Encyclopedia ofElectrochemistryof the Elements; Marcel Dekker: New York, 1984; Vol. XV, p 217. (23) o-Dinitrobenzene was selected because MV(PF& was insoluble in me-
thylene chloride. (24) Bard, A. J.; Lund, H. EncyclopediaofEIectrochemistryof the Elements; Marcel Dekker: New York, 1979; Vol. XIII, p 103. (25) Hoffman, M. Z.; Bolleta, F.; MORRi, __ L.; Hug, G. L. J . Phys. Chem. Ref. Data 1989, 18, 219. (26) Mandel, M.; Hoffman, M. Z. J . Phys. Chem. 1984, 88, 5632.
Inorganic Chemistry, Vol. 31, No. 15, I992 3233
(Biphenyl)platinum(II) Complexes Table 111. Stern-Volmer Constants and Quenching Rate Constants’
1@Ksv,b M-l k:, M-1 s-I complex 1.3 9.6 X 1 O l o [Wbph)(C~Hs)2S12~ [Wbph)(CH3CN)2lC 2.1 3.0 x 109 2.0 1.9 x 109 [Pt(bPh)(en)1 1.8 X 1Olo [P~(~P~)(PY)~I~ 5.6 v = +5%. k, error = &lo%.“o-Dini“ T = 23 & 1 OC. K ~ error trobenzene (E112 = -0.81 V); in methylene chloride. e Methyl viologen ( E l p = -0.42 V); in acetonitrile.
Table IV. Emission Spectral Curve-Fitting Parameters in Butyronitrile at 77 and 298 K‘
[Pt(b~h)(en)l [(Pt(bph)(CH&N)21 [Pt(bph)(py)21 77K 298K 77K 298K 71K 298K 20660 20408 20700 20450 20620 20325 20660 20430 20700 20455 20630 20370 1550 1570 1550 1575 1550 1560
Ecm.cm-l Em,cm-I hol,cm-I SI ho2,cm-1
0.90
1150
0.60 710
s 2
hw3,cm-I
0.25 375
s 3
ho4,cm-1
0.90
0.91
0.86
1150 0.62
1150
1150
0.60
0.60
720
720
720
0.29
0.26
0.27
0.83
1150 0.59 715
0.19
0.98
1150 0.62 715 0.27 375
375 375 375 375 0.28 0.28 0.28 0.28 0.28 495 355 530 AuII2,cm-I 350 350 530 Error limits are as_follows:temperature, f 2 K; Em, f20 cm-I; ho, f10 cm-I; S, f2%; A u I ~*5%. ,
sd_
0.28
Table V. Kinetic Decay Parameters#
h
kl, s-I k2, s-l AE2,
[Pt(b~h)(en)l [Pt(bph)(py)21 (7.6 f 0.5) X lo4 (7.0 f 0.1) X lo4 (1.1 f 0.8) X l O I 3 (3.8 A 0.5) X 10” 2490* 5 2280 5
*
[Pt(bph)(CH3CN)21 (6.5 & 0.1) X lo4 (2.5 0.1) X lo7 1220 5
cm-1 In 2-MeTHF; T = 1%-310 K; hx= 355 nm.
**
a
300: 200 22
21
20
19 18 17 ENERQY (1OOaa-n-1)
16
14
15
Figure 3. Corrected emission spectra of [Pt(bph)(CH3CN)2](+) in a
2-methyltetrahydrofuran glass at 77 K and in solution at room temperature. Spectra were calculated (-) using eq 4 and the spectral fitting parameters in Table IV.
Emission Spectral Fitting Parameters. The corrected emission spectra of [Pt(bph)(CH3CN)2]a t 77 and 298 K are shown in Figure 3. Band shapes calculated from spectral curve-fitting parameters described by eq 4 are also illustrated in Figure 3. The
I ( h w ) = I V ( h O ) / Z-~
C...C{((E, -
Yj(hUj)
VI.0
~ ( h w ~ ) ) /x~(s?/uj!) ,)~ x
In 2 ) ( ( h w - E,
- I..
vj-0
.., x
(si”//vj) x exp[-(4 x
+ v,(hw,) + ... + ~ ~ ( h w ~ ) ) / A ; ~ ,(4)~ ) ~ l )
emission profiles were analyzed by a four-mode, temperaturedependent Franck-Condon analysis based on the parameters Ew, (1, 2, 3, 4), hwi4, (1, 2, 3, 4), and A&p. In the equation, Em is the zero-zero energy, and hwi-j are respectively the electronic-vibrational frequency modes that vary in energy from medium- to low-energy frequency vibrations, Ai1/2 is the full width a t half-maximum for the individual vibronic contributors, and hw is the frequency of observation. In theemission spectrum, vibrational progressions can be seen at 77 K giving good initial estimates for hw values. The experimental spectra contained the theoretical features, but the resolution was less satisfactory, presumably due to the circular sample cells and the liquid nitrogen dewar windows. The data are collected in Table IV and can be summarized as follows. For the three complexes examined, parameters under the same heading have similar values. Thus, at 77 K, average values for the variables are as follows: Em = 20 660 cm-I, hwl = 1550 cm-I, hw2 = 1150 cm-I, hw3 = 715 cm-I, hug = 375 cm-I, SI= 0.88,S2 = 0.60, S 3 = 0.23,Sd = 0.28, and A&/2 = 352 cm-I. The similarity of values for a given variable is expected, since the excited electron resides in the ?r* energy
1 ,
140
I80
’
&O
’
260
’
3bO
Temperature (K)
Figure 4. Plot of 1/ T vs Tfor [Pt(bpy)(en)] in 2-methyltetrahydrofuran. The line is the result for the best fittingof eq 5 to the experimentalpoints.
level of the bph ligand. The correlation of S values with hw values is also reasonable. As S decreases in magnitude, the emission intensity decreases and becomes distributed among a larger number of lower energy modes. Temperature-DependentLifetime. Rate Constants for ExcitedState Decay. Kinetic decay parameters are collected in Table V. Temperature-dependent lifetimes were fit to eq 5 in order to ( T ( T))-’
= k, + k, exp(-AE,/RT)/[
1
+ exp(-AE,/RT)] (5)
obtain kl, kt, and AEz values.27 As illustrated in Figure 4, satisfactory fits of the data were obtained. The experimental data can be summarized as follows. Within the series, kl is several orders of magnitude (3-1 1) smaller than kZ. The values for AEz change by about a factor of 2 from 1220 to 2490 cm-I.
Discussion The goal of the work was to study the photochemical and photophysical properties of a new series of complexes based on platinum(1I). The key in the study was to fabricate molecules with high-energy emitting states. We took advantage of the fact that platinum(I1) is known to form complexes with metal-carbon bonds,28 which raise the energy of the ligand field state due to (27) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.;Meyer, T. J. J . Phys. Chem. 1986, 90, 3722. (28) Hartley, F. R. The Chemistry of Platinum and Palladium; Halsted (John Wiley and Sons, Inc.): New York, 1973.
3234 Inorganic Chemistry, Vol. 31, No. 15, 1992
strong u-bond interaction. This then provides a large energy spacing between the filled d orbitals and the du* state and, hence, the opportunity to fill this void with unfilled orbitals on ligands capable of emitting energy when populated. The biphenyl dianion was chosen as the chromophoric ligand for two reasons. First, it was possible to prepare 2,2’-dibromobiphenyl, convert it to 2,2’-dilithiobiphenyl by metal-halogen exchange, and then coordinate it to platinum(I1) as a bidentate chelating ligand through the 2,2’-carbon a t o m ~ . ~Second, ~ J ~ it contained an empty r* energy level with energy in the spacing between the filled d and unfilled du* states. Finally, to ensure that thefilleddorbitals were the HOMOand theunfilledr*(bph) orbitals were the LUMO, the other ligand@) chosen for coordination to platinum(I1) contained empty orbitals that were not located within the dd energy spacing. Optical transitions near 330 nm were observed from the HOMO to the LUMO. The transitions were assigned as d r r*(bph), in agreement with previously published results for similar c0mpounds.6-~The energy of the transition remained relatively fixed for the series of complexes. However, redox potentials for the irreversibleoxidation [Pt(bph)Lz] [Pt(bph)L2]+did change by over 0.7 V in the series [Pt(bph)(C2H&3]2 > [Pt(bph)(CH3CN)2] > [Pt(bph)(py)z] > [Pt(bph)(en)]. This variation and the fact that the optical transition remains at thesame energy would indicate that the energiesof the r* levels change in concert with the energy of the ground state. The emission spectra of the complexes shown in Figure 1 are basically identical to one another, indicating that emission occurs from a common origin. The highly resolved vibronic structure is typical of a ligand-centered process.6b In addition, the vibronic structure is similar to the behavior displayed by biphenyl itself, H2bph,but is better resolved. In the past, better resolution was achieved by near planarity of the phenyl which is the expected result of bidentate coordination. On the basis of the clearly resolved band structure, the emission can be assigned to a 3LC (ligand-centered) state with a small amount of MLCT character, which manifests itself in a loweringof radiative lifetimes from the millisecond time domain to the microsecond time scale. There is no experimentalevidence for the presence of the )MLCT state, which presumably lies at higher energy than the 3LC state. Similar observations have been reported for rhenium(1) tricarbony1 complexes.30 The results of emission spectral curve fitting of the complex give insight into excited-state structure. The results in Table IV obtained at room temperature and at 77 K were nearly the same, except at room temperature Em was red-shifted 150-300 cm-l, hwl was red-shifted 10-25 cm-I, and A;lpwas 15Ocm-1larger. The vibrationalprogressionsobtained from emission spectra were in agreement with data obtained from resonance Raman spectra for H2bphand Hbph-.j1J2 The displacementsof the modes have been determined by normal-coordinate analysis.33 The results indicate that vibrational coupling is largely distributed over the whole H2bphmolecule and atoms move preferentially all together. A comparison of similar frequencies found from emission curve fitting to those reported from resonance Raman spectroscopy for H2bphand Hbph- are listed in Table VI. We attempted to obtain resonance Raman spectra for [Pt(bph)L2] complexes as well.34 Due to solubility limitations, only one mode located at 1584 cm-I was observed for [Pt(bph)(en)] in methanol and 2-methyltetrahydrofuran and for [Pt(bph)(CH&N)2] in acetonitrile. The similarity of this value to the 1570 cm-l frequency found by the
Blanton et al. Table VI. Comparison of Vibrational Frequencies for [Pt(bph)L1] Complexes to Those for H2bph and Hbphv, cm-1
[Pt(b~h)Lzl’
1
1570 1150 720 375
2 3 4
Hbph1587 721 327
Hzbphc 1605 1190 743 314
a This work. Reference 28. Dewar, M. J. S.;Trinajstic, N. Czech. Chem. Commun. 1970, 35, 3136.
-
-
-
(29) Momicchioli, F.; Bruni, M. C.; Baraldi, I. J . Phys. Chem. 1972, 76, 3983. (30) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Striech, J.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1990, 29, 4335. (31) Takahashi, C.; Maeda, S. Chem. Phys. Len. 1974, 24, 584. (32) Naqvi, K. R.; Donatsch, J.; Wild, U.P. Chem. Phys. Lert. 1975,34,285. (33) Zerbi, G.; Sandroni, S. Spectrochim. Acro 1968, 24A, 511. (34) The resonance Raman spectrophotometer used was an ISA UlOOO instrument located at The University of North Carolina at Chapel Hill.
I
P
I
W
Nuclear coordinate Figure 5. Proposed ordering of energy levels and representation of hE2. 3LC is the luminescing excited state.
emission curve-fitting routine is remarkable and suggests that the resonance Raman and luminescence techniques allow one to measure the same phenomenon. One would expect the metal center to alter the energy of the modes. However,a comparison of the force constantsfor C(sp2)C(sp2) (9.60 mdyn/A), C(sp2)-H (4.6 m d ~ n / A ) , ~and ’ M-C (0.8 mdyn/A)’6 would indicate that, to a first approximation,the bph ligand may vibrate almost as if the metal were not present. This is certainly the conclusion one makes when comparing the similarity of vibrational frequenciesgiven for each mode in Table VI. The most likely exceptions to this postulateare the low energy modes, where the presence of metal-ligand vibrations are expected to contrib~te.9~ The proposed ordering of energy levels based on absorption, emission, and temperature-dependent lifetime studies is shown in Figure 5 . Upon absorption of energy, the3LC state is populated and undergoes radiative and nonradiative decay to the groundstate or energy transfer to another excited state by thermal means. The barrier to energy transfer, AEz, varies from 1220 to 2490 cm-1, depending on the complex. The physical significance of AE2and rates of energy transfer within the excited-state manifolds has been discussed for platinum( 11) complexes3*and rutheniu(35) Drago, R. S.Physical Merhods in Inorganic Chemistry;Reinhold: New York, 1965; p 191. (36) Timey, A. Inorg. Chem. 1979, 18, 2502. (37) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1963. (38) Barigelletti, F.;Sandhi, D.; Mastri, M.; Balzani, V.; von Zelewsky, A.; Chassot, L.; Jolliet, P.; Maeler, U. Inorg. Chem. 1988, 27, 3644.
(Biphenyl)platinum(II) Complexes
Inorganic Chemistry, Vol. 31, No. 15, 1992 3235
m(I1) heter~cycles,~~J~-@ and a similar interpretation can be used here. In ruthenium(I1) systems, A& was attributed to the energy needed to thermally populate a ligand field state from the emitting state. The probe for the existence of the ligand field state was photosubstitution in the presence of the C1- ion. Unfortunately, [Pt(bph)Lz] complexesunderwent a dark reaction in the presence of the C1- ion. Thus, it was not possible to probe the nature of the thermally assessible state in the case of the [Pt(bph)L2] complexes. Tentatively, it has been assigned to a dd state; but it may also be a higher energy 3LC state. Recently, we reported4I a correlation in a series of ruthenium( 11) polypyridyl complexes between the preexponential factor (k) and the energy term (AE)obtained by using eq 5 . This trend appears general, as noted here by the correlation between kz and AE2. If k2 is interpreted as the Arrhenius preexponential factor, then entropic arguments related to the higher density ofvibrational
states at the higher energy barrier crossovercan be used to account for the trend.41 The energy gap between the emitting state and the ground state is approximately 2.6 eV in the [Pt(bph)L2] complexes. Typical values for ruthenium(II), osmium(II), and rhenium(1) are less than 2.0 eV.1 Energy gaps in the 2.6-eV regime have been obtained for a few other complexes. Some examples are iridium phenylpyridine platinum dicyano complexes,11J2and some rhodium complexes.43Depending on groundstate redox potentials, rather substantial excited-state redox potentials can be achieved. For [Pt(bph)Lz] complexes, the approximate [Pt(bph)Lz]+/O*potential was-1.5 V. In conclusion, the design of molecules with large emission energy gaps may be useful for energy conversion studies when a large driving force is necessary to effect oxidation or reduction of substrates.44
(39) (a) Allen,G. H.; White, R. P.; Rillema, D. P.; Meyer,T. J. J . Am. Chem. SOC.1984,106,2613. (b) Rillema, D. P.;Taghdiri, D. G.; Jones, D. S.; Kellor, C. D.; Worl, L. A.; Meyer, T. J.; Levy, H. A. Inorg. Chem. 1987, 26, 578. (c) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.;Meyer, T. J . J . Phys. Chem. 1986,90,3722. (d) Durham, B.;Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J . Am. Chem. SOC.1982,104,4803. (e) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983,105,5583. (f) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (40) (a) Barigelletti, F.; Juris, A.; Belser, P.;von Zelewsky, A. J . Phys. Chem. 1987, 91, 1095. (b) Juris, A.; Barigelletti, F.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1985, 24, 202. (41) Rillema, D. P.; Blanton, C. B.; Shaver, R. J.; Jackman, D. C.; Boldaji, M.; Bundy, S.;Worl, L. A,; Meyer, T. J. Inorg. Chem. 1992,31, 1600.
Acknowledgment. We thank theofficeof Basic Energy Science of the Department of Energy under Grant No. DE-FGO584ER13263 for support, Johnson Matthey for the platinum source compounds on loan, and Lithco, Inc., a Division of FMC,for the alkyllithium compounds. (42) Dedeian, K.; Djurovich, P. I.; Garces, F. 0.;Carlson, G.; Watts, R. J. Inorg. Chem. 1991, 30, 1685. (43) Nishizawa, M.; Suzuki, T. M.; Sprouse, S.;Watts, R. J.; Ford, P. C. Inorg. Chem. 1984, 23, 1837. (44) Watts, R. J. Comments Inorg. Chem. 1991, I I , 303.
